Geothermal system utilizing supplemental ground heat from drainage fields

ABSTRACT

Geothermal ground coils are installed and embedded in fill at the bottom of excavations, made for drainage fields associated with septic and other effluent systems, prior to the addition of the drainage field sanitary fill. This configuration eliminates the need for expensive bore holes and provides a ground heat source that is at a higher temperature than natural ground formations due to the supplemental heat imparted to the ground by the drain field effluent Fouling of the outside heat transfer surfaces of the ground coils is also avoided because of the clarifying and filtering action of the drain field fill. This provides an improved ground source heat pump system wherein the installation costs are reduced arid the capacity and efficiency of the system are improved.

REFERENCES CITED [REFERENCES BY]

U.S. Patent Documents 2,563,262 August, 1951 Moore  126/344 4,448,347 May, 1984 Dunstan  165/909 5,477,914 December, 1995 Rawlings 165/45 5,533,355 July, 1996 Rawlings 165/45 5,730,208 March, 1998 Barban 165/45 5,738,164 April, 1998 Hilderbrand 165/45 5,758,717 June, 1998 Grossman 165/47

FIELD OF INVENTION

The present invention relates to heat recovery systems and methods, and more particularly pertains to a system and method for recovery of heat from the ground affected by drainage fields.

BACKGROUND OF THE INVENTION

Several prior art patents have utilized the earth as a source of heat and as a heat sink. Commercial geothermal energy systems are available and have been in use for several years. These systems normally utilize a water feed heat pump for both heating and cooling. The circulating fluid that feeds the heat pump flows through tubes that are buried deep in the ground and the circulating fluid uses the ground surrounding the tubes as both a heat source and a heat sink depending on whether it is being used for heating or cooling. U.S. Pat. No. 5,738,164 (Hilderbrand) describes such a system using bore wells in the ground. U.S. Pat. No. 5,533,355 (Rawlings) describes a ground source heat system. These and similar patents all depend on the normally occurring temperatures and conductivity of the natural ground formations at the location where the geothermal system coils are installed. U.S. Pat. No. 4,448,347 (Dunston) discloses a system whereby heat is recovered from household wastewater and U.S. Pat. No. 5,730,208 (Barban) discloses a system whereby heat is recovered from the bio-thermal heat of septic tanks. These and similar patents U.S. Pat. No. 2,563,262 (Moore) make use of tanks that collect the wastewater from the building or use the septic tank itself and use a heat exchanger in the separate tank or the septic tank to recover heat from the wastewater. In practice, the heat transfer efficiency of these systems rapidly decreases because wastewater is inherently dirty and causes a buildup of material on the heat exchange surfaces. This fouling of the heat exchange surfaces reduces the heat transfer coefficient between the wastewater and the circulating fluid inside the coils and the amount of heat recovered by these systems drops off very quickly and renders them as being impractical.

In northern climates the primary use for these systems would be for heating buildings with air conditioning as a secondary use. The majority of geothermal systems are installed by drilling vertical holes into the ground and installing polymer or other types of coils into the holes using a heat exchange grout to improve heat transfer from the ground to the fluid inside the coils. These geothermal systems are normally used because the extra costs for installing the system can be offset by a reduction in the energy costs used to operate the system and the reduction of greenhouse gases.

Typical published data indicate that for a 2000 sq ft dwelling, a geothermal system will save about $1000 per year in energy costs. Of course this will vary with the climate and cost of fuel. However in actual practice, during the coldest periods of the year, the heat load obviously increases and the heat removal requirement from the ground in contact with the geothermal coils also increases but the natural heat recovery by the ground surrounding the coil is too slow to maintain the normal ground temperature. This phenomenon reduces the ground temperature near the coil and in turn reduces the heating capacity of the geothermal system. During these periods, the heating system of the building normally incorporates electric heating coils to make up for the shortfall of the geothermal system. The energy costs increase because of the auxiliary electric heating requirements. This greatly reduces the return on investment used to justify the geothermal system. As an example, if we assume the installation of a geothermal system increased the cost by $10,000 over the cost of a fossil fuel system, the $1000 per year savings would represent a 10% annual return on the $10000 extra cost. However if the auxiliary electric heaters are required for substantial periods the operating savings can be reduced to $500 per year resulting in only a 5% return. The invention of this patent is to provide a geothermal system whereby the installation costs are substantially reduced and the operating costs are also substantially reduced over normal geothermal practice. The heating capacity of the system is substantially increased over the same size systems not designed using the teachings of this patent. The practice of this invention substantially improves the return on investment for geothermal energy systems.

SUMMARY OF THE INVENTION

The present invention relates to a method of increasing the overall energy efficiency as well as reducing the installation costs of geothermal systems. The subject method comprises of installing the geothermal ground coils at the bottom of the excavations made for septic systems and similar drainage field applications to eliminate the extra cost of drilling bore holes or separate excavations. After the ground coils are placed at the bottom of the drainage field excavation, sanitary fill is used to surround and cover the coils and build up the drainage fields before the effluent distribution system is installed and covered. When the septic or other effluent system is operational, the supplemental heat from the effluent raises the temperature of the drainage field so that when heating is required, the temperature of the circulating fluid in the geothermal coils will be higher than if they were installed in natural ground formations. This higher circulating fluid temperature results in a substantial increase in energy efficiency. Also because the effluent is clarified by percolation and filtration, by the drainage field, buildup and fouling on the surface of the ground coils is avoided.

The preferable application of this invention would be where there is new building construction in rural or suburban areas where there is no central sewer system and where private septic systems are used. In these situations, the septic system requires a large drainage field area where perforated distribution pipes are installed horizontally and the liquid discharged from the septic tank flows into a header and this liquid then flows into a network of horizontal perforated pipes. The liquid is distributed over a designated drain field area where it percolates into the ground and is subjected to further bacterial digestion and filtration to purify the liquid. In the preferred application after the excavation for the septic drain field is completed, and before the required fill is added, the geothermal piping can be installed horizontally at the bottom of the excavation. The septic fill and the septic fields could then be installed in accordance to code. Since no drilling is involved as would be required in vertical geothermal systems, the very expensive drilling costs are eliminated. Because the geothermal coils are installed horizontally at the bottom and are completely exposed, the installation costs are greatly reduced. More coils can be added at a lower cost. After the geothermal coils are installed, the fill used for the septic system is added above the geothermal coil fill and the distribution pipes for the septic effluent are installed in the normal manner. The number and total length of the perforated pipes, the area of the drain field, and the depth of the fill for the septic system is dictated by the local building codes. The fill used for the geothermal coils should not be used to modify these codes. Once the building is occupied, the septic system will become operational and the liquid exiting the septic tank will be distributed across the drainage field. The liquid exiting the septic tank is inherently warmer than typical ground temperatures associated with standard geothermal systems so that the drainage fields will be continuously maintain at a higher temperature than the natural ground formations.

As a further enhancement of the system, in order to improve the efficiency of the system when cooling is required, one or more normal geothermal ground coils can be installed in ground which is outside the drainage field in order to take advantage of the lower temperature in the natural ground formations. These normal ground coils would preferably be in series with the drainage field coils. In the summer, when cooling is required, the circulating fluid would first go to the coils in the drainage field and then would flow into the coils outside the drainage fields, which would further reduce the temperature of the fluid in the coils before it flows to the cooling portion of the heat pump. In the winter, when heating is required, the flow of the circulating fluid would be reversed so that it would flow from the outlet of the heating portion of the heat pump to the normal ground coil in the natural ground formation where it will cause the temperature of the circulating fluid to rise. The circulating fluid would then flow into the drainage field ground coils where the temperature would further increase before flowing to the inlet of the heating portion of the heat pump. This higher circulating fluid temperature increases the capacity and efficiency of the geothermal heating system.

Another method to meet cooling requirements with the higher circulating fluid temperature from the drainage field coils would be that because installation costs are reduced, extra coils can be installed in parallel. The volumetric flow can then be increased during cooling to offset the higher fluid temperature and still absorb the required cooling load.

In most northern climates ground temperatures typically vary from 50-55 degrees Fahrenheit depending on the season. FIG. 2 is a graph of the way normal ground temperature will vary from month to month compared to the minor variations of the ground temperature in the drainage field. This illustrates that normal ground temperature is usually lowest during the winter months when higher ground temperatures are desirable. The temperature of the ground in the drainage fields will not drop very much in the winter because of the heat recovery from the discharge of the warm effluent from the building and the heat released as a result of bacterial digestion in the septic tank and fields.

The liquid discharged from septic tanks is typically 5-10 degrees Fahrenheit higher than normal ground temperatures. Since the liquid distributed by the septic systems perforated pipes is subject to further exothermal bacterial digestion in the drain field, the ground temperatures under the drain fields will be effected not only by natural geothermal heat flux, but additional heating is achieved from the sensible heat from the waste liquids from the building and from the heat produced by the bacterial digestion in the septic tank and the drainage fields. Since higher moisture increases the heat conductivity of the soil, the steady flow of liquid to the septic drainage fields increases the rate of heat recovery in the ground in the vicinity of the geothermal coils. The efficiency of the geothermal system is therefore maintained during the coldest months when it is needed most. This reduces or completely eliminates the need for supplemental electric heating during the coldest months, thereby reducing or eliminating the additional cost for supplemental electric heating. The higher ground temperatures also reduce the heating costs during the entire heating season.

Although the preferred method is the installation of horizontal geothermal loops prior to the installation for the septic field in new construction or during the replacement of existing septic fields. The teachings of this patent can be used with existing septic systems and fields. With existing drainage fields the geothermal loops could be installed in holes drilled diagonally or vertically so that portions of the loops are in contact with the ground that is affected by the drainage fields. These coils must be installed so as not to interfere with the distribution pipes used to distribute the effluent to the drainage fields.

DRAWINGS DESCRIPTION OF DRAWINGS

FIG. 1 is a graph, which illustrates the relative heat conductivity of various types of soil and how the conductivity varies with moisture.

FIG. 2 is a graph, which illustrates how the temperatures of naturally ground formation and the temperature in drainage fields vary during the year.

FIG. 3 is a schematic cross sectional view of an embodiment of the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THIS INVENTION

Referring to FIG. 3, the waste stream from building 1 flows through the waste stream drain pipe 2 into the septic tank 3 where anaerobic bacterial digestion takes place. The effluent form the septic tank, 3 flows through the effluent outlet line 4 into the effluent distribution lines 5 and is distributed into the sanitary fill drainage field 6. The effluent percolates through the effluent drainage field 6 where it is subjected to further bacterial digestion and is clarified by the filtering action of the sanitary fill in the drainage field 6. As the effluent percolates through the drainage field 6, the temperature of the field rises due to the sensible heat of the effluent and heat from the biological digestion in the septic tank 3 and in the drainage field 6. This in turn causes the temperature of the fill 7 below the drainage field to also rise.

A circulating water feed heat pump 8 is used to provide heating and air conditioning for building 1. During the heating season, water or any other fluid in the circulating loops flows from the heat pump 8 to the connecting line 9 into the supplemental geothermal ground loop 10, which is outside the drainage field and in a natural ground formation 13. Transfer of the heat from the ground to the fluid inside the supplemental ground coil 10 raises the temperature of the fluid. The fluid then flows into the geothermal drainage field coils 11, which are embedded in the fill 7. Since the temperature of the fill 7 is higher than the ground in contact with the supplemental coil 10 the temperature of the fluid inside the drainage field coils 11 will further rise. The fluid at this higher temperature would then flow through the connecting line 12 to the heat pump 8. The higher temperature of the fluid results in substantial energy savings during the heating season.

During the cooling season, the direction of flow in the circulating loop is reversed so that the circulating fluid would flow into the geothermal drainage field coils 11 first to reduce the temperature of the fluid from the cooling cycle of the heat pump 8 and then to the supplemental geothermal coil 10 where the temperature of the circulating fluid would be further reduced before it flows to the heat pump 8 which is operating in the cooling mode.

This configuration of the preferred embodiment of the invention results in lower installation costs, higher ground heat transfer with minimum fouling of heat exchange surfaces and provides an increase in the efficiency and capacity of geothermal systems for both heating and cooling. 

1. A system whereby the ground loops used in domestic, commercial and industrial geothermal systems are installed in such a manner as to take advantage of the natural geothermal properties of the ground as well as take advantage of the supplemental heat the ground receives from fluids discharged into the drainage fields which are associated with septic systems, dry wells, catch basins, or any other effluent or fluid which eventually is distributed into the ground and affects the temperature of the ground in the vicinity of geothermal loops.
 2. A system as described in claim 1 whereby horizontal ground loops are installed below and before the installation of new or replacement drainage fields which are integral to domestic, commercial or industrial waste treatment systems thereby eliminating the cost of drilling expensive bore holes or separate excavations for installation of the geothermal coils.
 3. A system as described in claim 1 whereby the effluent flow to the drainage fields originates from hospitals, schools, prisons, airports or any military, government or institutional site or any other type of installation.
 4. A system as described in claim 1 whereby the geothermal ground coils are installed and are connected preferably in series with either horizontal or vertical geothermal ground coils installed in ground outside the influence of the higher temperature drainage fields. During the period when cooling is required, fluid in the geothermal ground loops would flow from the outlet of the air conditioning portion of the heat pump then to the ground loops installed in the drainage fields according to claim 1 and then to the ground loops outside the influence of the drainage fields so that the temperature of the circulating fluid in the ground loop would be further lowered before flowing to the inlet of the air conditioning portion of the heat pump.
 5. A system according to claim 1 and claim 4 whereby when heating is required the flow of the circulating fluid is reversed so that the flow from the outlet of the heating portion of the heat pump goes to the regular ground coil first and then to the coils installed according to claim 1 where the temperature is further increased before it flows to the inlet of the heating portion of the heat pump.
 6. A system as described in claim 1 whereby because of the low installation costs extra geothermal ground coils are added in parallel with ground coils installed according to claim 1 so that during periods when cooling is required the higher temperature of the circulating fluid due to the higher temperature of the drainage fields would be offset by the higher volumetric flow from the extra parallel coils. The extra volume of circulating fluid would remove the required heat load at a lower temperature differential.
 7. A system as described in claim 1 whereby fouling of the heat exchange outside surface of the geothermal coils is minimized by allowing the fill of the drainage field to further digest, filter and clarify fluids before they come into contact with the heat transfer surfaces of the geothermal ground coils.
 8. A system as described in claim 1 whereby geothermal ground coils are installed horizontally, diagonally or vertically in any combination in new or existing drainage fields so as to take advantage of the supplemental heat imparted into the ground as a result of fluid flows from septic systems, drywells, catch basins and other drainage systems. 